1. Field of the Invention
The present invention relates to an optical pickup for optically recording information on the recording surface of a recording medium such as an optical disk by focusing laser light onto the recording surface, and/or for playing back the information written (recorded) on the recording surface by detecting light reflected at the recording surface of the recording medium. More particularly, the invention relates to a thin optical pickup having a hologram mirror for redirecting the optical axis of incident light from a light source so that it is perpendicular to the recording surface of a recording medium.
2. Description of the Related Art
In recent years, use of optical disks having high packing densities and capable of storing large volumes of information has been spreading in a variety of fields. Because of its ability to record and play back information in non-contacting manner, coupled with the compatibility of the medium, the optical disk is particularly promising as an optical file storage or external storage medium for a computer.
Information is recorded on or played back from such optical disks by means of an optical pickup using laser light. Depending on the type of medium, recorded information can be erased or overwritten. For optical pickups for this type of medium, reducing the size and weight of optical pickups has been a major problem in achieving high access speeds.
Conventional optical pickups that address this problem include the ones disclosed in Japanese Laid-Open Patent Publication No. 2-71442 and U.S. Pat. No. 4,918,679. In either example, size and weight reductions are achieved by reducing the height of the optical pickup by using a hologram mirror.
FIG. 12A shows a conventional optical pickup using a hologram mirror. A laser beam emitted from a laser diode not shown is collimated by a collimator lens 6. The collimated beam x is directed to a hologram mirror 7. The light incident on the hologram mirror 7 is diffracted by the reflective holographic optical element 7a formed on the incident surface of the hologram mirror 7, and among diffracted light +1st order diffracted light is directed in the direction perpendicular to the surface of a disk 10. Then, light from the hologram mirror 7 is converged by an objective lens 9 onto the surface (recording face) of the disk 10.
FIG. 12B is a cross-sectional view showing a structural example of the hologram mirror 7. The reflective holographic optical element 7a formed on the incident surface of the hologram mirror 7 is a grating of straight lines formed at pitch d, and has a sawtooth-like cross section. The holographic optical element 7a, with sawtooth-like grooves formed at pitch d on the substrate surface of the hologram mirror, is coated with a reflective film 18. Using the hologram mirror 7 of such a structure, the overall thickness of the optical pickup measured in the direction perpendicular to the disk is reduced, compared with an optical pickup that uses a mirror the face of which is tilted with respect to the optical axis of light by 45 degrees (hereinafter, referred to as 45-degree tilted mirror). The height of the optical pickup can thus be reduced.
In the conventional hologram mirror 7 having the structure shown in FIG. 12B, the first-order light diffraction efficiency of the reflective holographic optical element 7a is dependent on the groove shape of the holographic optical element 7a on the light incident side. In the conventional structure, since the grooves are covered with the reflective film 18, not only the effective groove depth of the holographic optical element 7a is reduced, but the groove shape is also rounded, as shown in FIG. 12B. Consequently, it has been difficult to obtain the desired diffraction efficiency. Furthermore, since minimizing the ill effect of the reflective film on the groove shape demands a very sophisticated film deposition technique, formation of the reflective film has involved an extremely difficult technology, which has been a bottleneck in increasing the productivity of optical pickups.
Furthermore, the reflective film surface may be partially deteriorated, deformed, or degraded with time. Such phenomena produce effects equivalent to causing the groove shape to change. Accordingly, there has been a fear that the diffraction efficiency may degrade or unwanted higher-order diffracted light may occur as a result of the reflective film's deterioration with time.